1. Field of the Invention
The invention relates to a mode-locked semiconductor laser for generating an ultra-short optical pulse train used for ultra-fast optical communication, and more particularly to optimization of mode-locked operation. The invention also relates to a method of driving the above-mentioned mode-locked semiconductor laser.
2. Description of the Related Art
There has been long expected to establish basic techniques for ultra-fast optical communication, such as technique for generating an optical pulse train close to Fourier transform limit and having a repetition rate in the range of tens of GHz, and technique for controlling an oscillation wavelength associated with a transmission wavelength.
As one of optical oscillators for accomplishing the above-mentioned techniques, there has been used a mode-locked semiconductor laser having a four-section electrode structure including a saturable absorber section, a gain section, a distributed Bragg reflector section for controlling an oscillation wavelength, and a phase control section for continuously controlling a wavelength, these four sections being integrated.
For instance, one of such mode-locked semiconductor lasers has been suggested in "Transform-Limited Optical Short-Pulse Generation at High Repetition Rate over 40 GHz from a monolithic Passive Mode-Locked DBR Laser Diode" written by Shin Arahira et al. in IEEE Photonics Technology Letters, Vol. 5, No. 12, December 1993, pp. 1362-1365. According to the article, a monolithic passive mode-locked distributed Bragg reflector laser diode was fabricated. An optical short-pulse train with a duration of 3.5 ps was generated from the laser at a high repetition rate of over 40 GHz. The time-bandwidth product was 0.43, and was quite close to the transform-limited value of a Gaussian waveform.
Another example of a mode-locked semiconductor laser is found in "Generation of 33 GHz stable pulse trains by subharmonic electrical modulation of a monolithic passively mode-locked semiconductor laser" written by T. Hoshida et al. in ELECTRONICS LETTERS, Vol. 32, No. 6, Mar. 14th, 1996, pp. 572-573. Subharmonic hybrid mode-locking of a monolithic semiconductor laser was reported. According to the article, 33 GHz pulse trains with reduced phase noise were obtained from the laser by modulating its saturable absorber electrically at cavity subharmonic frequencies of 16.5, 11 and 8.25 GHz.
Still another example of a mode-locked semiconductor laser is found in "Monolithic strained InGaAsP multiple-quantum-well lasers with integrated electroabsorption modulators for active mode locking" written by Kenji Sato et al. in Applied Physics Letters, Vol. 65, No. 1, Jul. 4th, 1994, pp. 1-3. The article reported active mode locking by monolithic lasers with integrated electroabsorption modulators using strained-InGaAsP multiple quantum wells. The electroabsorption modulator acted as a short optical gate when a sinusoidal voltage was driven at a deep bias point. Pulse widths as short as 2 ps was obtained at a repetition rate of 16.3 GHz for a 2.5-mm-long monolithic laser.
Yet another example of a mode-locked semiconductor laser is found in "Multifunctional Application of Monolithic Mode Locked Laser in (O)TDM Systems: Pulse Generation and Optical Clock Recovery" written by E. Lach et al., 22nd European Conference on Optical Communication--ECOC'96, Oslo, pp. 4.23-4.26. There was reported multifunctional operation of monolithic active/passive mode locked DBR laser for 4.times.10 Gb/s OTDM with performance at STM-64 frequency: nearly time-bandwidth limited (&lt;0.4) pulses (&lt;10 ps), large pulse extinction (&gt;20 dB) and optical clock recovery (10 GHz) from 40 Gb/s data.
Still yet another example of a mode-locked semiconductor laser is found in "Short Pulse Generation Using Multisegment Mode-Locked Semiconductor Lasers" written by Dennis J. Derickson et al. in IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 28, No. 10, October 1992, pp. 2186-2202. There has been suggested mode-locked semiconductor lasers incorporating rate multiple contacting segments. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation are integrated on a single semiconductor chip. The optimization of the performance of multisegment mode-locked lasers are accomplished in terms of material parameters, wavelength parameters, electrical parasitics, and segment length.
FIG. 1 illustrates one of conventional mode-locked semiconductor lasers. The illustrated mode-locked semiconductor laser includes a substrate 61 on which are formed a first cladding layer 52, waveguide layer 2, a second cladding layer 51, a cap layer 41, and an electrode 31 in this order. An electrode 32 is formed on a lower surface of the substrate 61. The waveguide layer 2 is comprised of a saturable absorber section 11, a gain section 12, a phase control section 16, and a distributed Bragg reflector section 15. An active layer 22 is formed in the saturable absorber section 11 and the gain section 12, and a non-absorptive waveguide layer 23 is formed in the distributed Bragg reflector section 15 and the phase control section 16.
The active layer 22 has a multiple quantum-well (QW) structure where well layers are made of InGaAs, and barrier layers are made of InGaAsP. The non-absorptive waveguide layer 23 has an absorption edge wavelength of 1.3 .mu.m at bulk of InGaAsP. A diffraction grating 26 is formed in the distributed Bragg reflector section 15 at a boundary between the non-absorptive waveguide layer 23 and the second cladding layer 51. The mode-locked semiconductor laser is covered at an end thereof with highly reflective coating material 7.
In the mode-locked semiconductor laser having the above-mentioned structure, a reverse bias voltage is applied to the saturable absorber section 11, and a current is applied to the gain section 12. As a result, mode-locking operation is carried out, and optical frequency bandwidth is restricted in the distributed Bragg reflector section 15. Hence, a longitudinal mode which does not contribute to the mode-locking operation is suppressed, resulting in that short pulse characteristic approximate to Fourier transform limit can be obtained.
In addition, it is possible to continuously control an oscillation wavelength in dependence on conditions for an injection current to the distributed Bragg reflector section 15 and the phase control section 16.
In general, a mode-locked semiconductor laser is required to be electrically controlled by a stabilizing high frequency power source in order to enhance accuracy in a repetition rate and stability in operation in the application thereof to optical communication. To this end, both a high frequency signal and a reverse bias voltage have been applied to a saturable absorber section in a conventional mode-locked semiconductor laser. In this process, the optimization of a high frequency circuit is necessary to be carried out, for instance, by reduction in a parasitic capacitance and/or impedance matching in order to enhance an efficiency with which high frequency signals are applied to a saturable absorber section.
However, even if a saturable absorber section were designed so that an efficiency with which high frequency signals are applied thereto, conditions such as a section length and a bias voltage may get out of an optimal range for the mode-locked semiconductor laser to conduct saturable absorber operation.